An axial-piston machine such as an axial-piston pump or motor generally comprises a rotatable cylinder drum formed with a plurality of angularly equispaced cylinder bores each of which receives a piston which is reciprocated parallel (substantially) to the axis of rotation of the cylinder drum as the latter is rotated.
The pistons are usually formed with an end which extends from the cylinder drum and engages an inclined or inclinable reaction surface along which this end of the piston can ride or with which this end of the piston is coupled.
In modern axial-piston machines of this type, the piston is formed at this end with a ball head which is engaged with a shoe riding along the reaction surface or received in a socket of the reaction surface, depending upon the type of axial-piston machine which is involved.
The axially reciprocatable pistons are provided with central bores which terminate at these ball heads to provide a fluid cushion between the end of the piston and the reaction surface or the shoe by means of which it is retained against bears upon the reaction surface. The bore terminates at the opposite end of the piston which is juxtaposed with the end of the cylinder provided with a port adapted to communicate with the valve or control surfaces of the machine upon which the cylinder rides. This surface may be formed by a plate or by an end wall of a housing in which the cylinder is journaled.
Thus fluid pressure is transmitted from the working compartment of the cylinder to the head of the piston so that the fluid transfer by the piston passage constitutes a lubricating and friction-relieving medium at the piston head.
It is known, in such machines, to provide a hollow piston which cooperates with the swash plate of the axial-piston machine and which is formed with a central post intended to reduce the mass of the piston. This central post is formed with the passage and only partially fills the interior of the piston. As a result, the piston is constituted by an outer piston member whose external wall is cylindrical and which is of relatively thin-wall construction to reduce the piston mass. Since it is disadvantageous to completely fill the piston with the fluid medium, for reasons which will be dealt with below, the post extending through the piston centrally forms a passage of limited volume and, at the same time, supports the opposite ends of the piston against one another and may provide support for the cylindrical wall portion of the outer piston member as well.
Such hollow pistons are used so as to reduce the combined weight of the rotating cylinder drum and the plurality of pistons reciprocating therein.
In order to maintain this rotating mass as low as possible, it is desirable to make the pistons completely hollow. However, if the pistons are also to serve to transmit fluid from the working end of the piston to the reaction end or spherical head thereof, a relatively large fluid volume can be formed in the interior of the piston. If the piston wall is thin, this fluid is pressurized during the rotation of the drum and tends to press the wall of the piston outwardly and distort the latter. This can cause the piston to seize in its cylinder bore. Furthermore, because the wall of the piston is alternately compressed and expanded, energy is consumed which reduces the efficiency of the axial piston machine.
To avoid this expansion and compression and, more generally, any distortion of the hollow piston, it has been proposed to provide a post within the piston and to form this post with a small diameter passage communicating between the working end and the reaction end of the axial piston.
Such a piston is described in German open application (Offenlegungsschrift) No. 2,364,725.
The piston described in this publication has an end cover which is provided at the working end of the piston, i.e. the end opposite the reaction end or spherical head, which is bonded to the remainder of the outer piston body in a plane lying perpendicular to the piston axis by pressure-friction welding. Pressure-friction welding is effected by pressing the cover element against the body at the faces between them and displacing the body or the cap relative to the other at such a velocity that the friction force generates sufficient heat to cause fusion of the two parts.
Since the cover also engaged the post, the contact surface between the centrally disposed post and the cover was at a smaller diameter than the contact surface between the outer body and the cover. As a result, the relative speed of the post and the cover was frequently too small to obtain a satisfactory press-friction welding of the post to the cover. As a consequence, a satisfactory seal between the two could not always be obtained and some leakage of liquid into the space surrounding the post frequently occurred. This unnecessarily increased the mass of the piston and, in addition, caused the distortion and compression/contraction phenomena discussed previously since the cover was provided with a bore which registered with the bore of the post.